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Chapter 6
Are Financial
Markets Efficient?

Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-2
Chapter Preview
• We examine the basic reasoning behind
the efficient market hypothesis—the ideas
that market prices reflect what we know.
We also examine empirical evidence
examining this idea. Topics include:
– The Efficient Market Hypothesis
– Evidence on the Efficient Market Hypothesis
– Behavioral Finance

Efficient Market Hypothesis
• Expectations equal to optimal forecasts implies
R 
Pt 1  Pt  C
Pt
(7)
P
t1
e
 P
t1
of
 Re
 Rof
Market equilibrium
(8)
Put (8) and (9) together: efficient market hypothesis
R
e

P
t1
e
 P
t  C
P
t
Re
 R*
(9)
Rof
 R*
(10)

• This equation tells us that current prices in a financial
market will be set so that the optimal forecast of a
security’s return using all available information
equals the security’s equilibrium return.
• Financial economists state it more simply: A security’s
price fully reflects all available information in an
efficient market.
Rof
 R*

• Why efficient market hypothesis makes sense
All unexploited profit opportunities eliminated
Efficient market condition holds even if there are
uninformed, irrational participants in market
If Rof
 R*
 P
t   Rof

If Rof
 R*
 P
t   Rof

until Rof
 R*

Rationale Behind the Hypothesis
• When an unexploited profit opportunity
arises on a security (so-called because, on
average, people would be earning more
than they should, given the characteristics
of that security), investors will rush to buy
until the price rises to the point that the
returns are normal again.

Rationale Behind
the Hypothesis (cont.)
• In an efficient market, all unexploited profit
opportunities will be eliminated.
• Not every investor need be aware of every
security and situation, as long as a few
keep their eyes open for unexploited profit
opportunities, they will eliminate the profit
opportunities that appear because in so
doing, they make a profit.

Stronger Version of the Efficient
Market Hypothesis
• Many financial economists take the EMH one
step further in their analysis of financial markets.
Not only do they define an efficient market as one
in which expectations are optimal forecasts using
all available information, but they also add the
condition that an efficient market is one in which
prices are always correct and reflect market
fundamentals (items that have a direct impact
on future income streams of the securities)

Market Hypothesis (2)
• This stronger view of market efficiency has
several important implications in the academic
field of finance:
1. It implies that in an efficient capital market, one
investment is as good as any other because the
securities’ prices are correct.
2. It implies that a security’s price reflects all
available information about the intrinsic value of
the security.

Market Hypothesis (2)
3. It implies that security prices can be used by
managers of both financial and nonfinancial
firms to assess their cost of capital (cost of
financing their investments) accurately and
hence that security prices can be used to help
them make the correct decisions about whether
a specific investment is worth making or not.

Evidence on Efficient
Market Hypothesis
• Favorable Evidence
1. Investment analysts and mutual funds don't beat
the market
2. Stock prices reflect publicly available info:
anticipated announcements don't affect stock price
3. Stock prices and exchange rates close to random
walk; if predictions of ∆P big, Rof > R*  predictions
of ∆P small
4. Technical analysis does not outperform market

Evidence in Favor
of Market Efficiency
• Performance of Investment Analysts and
Mutual Funds should not be able to
consistently beat the market
– The “Investment Dartboard” often beats
investment managers.
– Mutual funds not only do not outperform the market on
average, but when they are separated into groups
according to whether they had the highest or lowest
profits in a chosen period, the mutual funds that did
well in the first period do not beat the market in the
second period.

Evidence in Favor
• Performance of Investment Analysts and
Mutual Funds should not be able to
consistently beat the market
– Investment strategies using inside information is the
only “proven method” to beat the market. In the U.S., it
is illegal to trade on such information, but that is not
true in all countries.

Evidence in Favor
• Do Stock Prices Reflect Publicly
Available Information as the EMH
predicts they will?
– Thus if information is already publicly available,
a positive announcement about a company will
not, on average, raise the price of its stock
because this information is already reflected in
the stock price.

Evidence in Favor
• Do Stock Prices Reflect Publicly
Available Information as the EMH
predicts they will?
– Early empirical evidence confirms: favorable
earnings announcements or announcements of
stock splits (a division of a share of stock into
multiple shares, which is usually followed by
higher earnings) do not, on average, cause
stock prices to rise.

Evidence in Favor
• Random-Walk Behavior of Stock Prices that
is, future changes in stock prices should, for
all practical purposes, be unpredictable
– If stock is predicted to rise, people will buy to
equilibrium level, if stock is predicted to fall, people will
sell to equilibrium level (both in concert with EMH)
– Thus, if stock prices were predictable, thereby causing
the above behavior, price changes would be near zero,
which has not been the case historically

Evidence in Favor
• Technical Analysis means to study past stock
price data and search for patterns such as
trends and regular cycles, suggesting rules
for when to buy and sell stocks
– The EMH suggests that technical analysis is a waste
of time
– The simplest way to understand why is to use the
random-walk result that holds that past stock price
data cannot help predict changes
– Therefore, technical analysis, which relies on such
data to produce its forecasts, cannot successfully
predict changes in stock prices

Evidence on Efficient
Market Hypothesis
• Unfavorable Evidence
1. Small-firm effect: small firms have abnormally
high returns
2. January effect: high returns in January
3. Market overreaction
4. Excessive volatility
5. Mean reversion
6. New information is not always immediately
incorporated into stock prices
• Overview
1. Reasonable starting point but not whole story

Evidence Against Market Efficiency
• The Small-Firm Effect is an anomaly. Many empirical
studies have shown that small firms have earned
abnormally high returns over long periods of time,
even when the greater risk for these firms has been
considered.
– The small-firm effect seems to have diminished in recent years
but is still a challenge to the theory of efficient markets
– Various theories have been developed to explain the small-firm
effect, suggesting that it may be due to rebalancing of portfolios by
institutional investors, tax issues, low liquidity of small-firm stocks,
large information costs in evaluating small firms, or an
inappropriate measurement of risk for small-firm stocks

• The January Effect is the tendency of
stock prices to experience an abnormal
positive return in the month of January
that is predictable and, hence,
inconsistent with random-walk behavior

• Investors have an incentive to sell stocks before the end
of the year in December because they can then take
capital losses on their tax return and reduce their tax
liability. Then when the new year starts in January, they
can repurchase the stocks, driving up their prices and
producing abnormally high returns.
• Although this explanation seems sensible, it does not
explain why institutional investors such as private pension
funds, which are not subject to income taxes, do not take
advantage of the abnormal returns in January and buy
stocks in December, thus bidding up their price and
eliminating the abnormal returns.

• Market Overreaction: recent research suggests that
stock prices may overreact to news announcements
and that the pricing errors are corrected only slowly
– When corporations announce a major change in earnings, say, a
large decline, the stock price may overshoot, and after an initial
large decline, it may rise back to more normal levels over a period
of several weeks.
– This violates the EMH because an investor could earn abnormally
high returns, on average, by buying a stock immediately after a
poor earnings announcement and then selling it after a couple of
weeks when it has risen back to normal levels.

• Excessive Volatility: the stock market appears to display
excessive volatility; that is, fluctuations in stock prices
may be much greater than is warranted by fluctuations in
their fundamental value.
– Researchers have found that fluctuations in the S&P 500 stock
index could not be justified by the subsequent fluctuations in the
dividends of the stocks making up this index.
– Other research finds that there are smaller fluctuations in stock
prices when stock markets are closed, has produced a consensus
that stock market prices appear to be driven by factors other
than fundamentals.

• Mean Reversion: Some researchers have
found that stocks with low returns today tend
to have high returns in the future, and
vice versa.
– Hence stocks that have done poorly in the past are
more likely to do well in the future because mean
reversion indicates that there will be a predictable
positive change in the future price, suggesting that
stock prices are not a random walk.
– Newer data is less conclusive; nevertheless, mean
reversion remains controversial.

• New Information Is Not Always Immediately
Incorporated into Stock Prices
– Although generally true, recent evidence suggests that,
inconsistent with the efficient market hypothesis, stock
prices do not instantaneously adjust to profit
announcements.
– Instead, on average stock prices continue to rise for
some time after the announcement of unexpectedly
high profits, and they continue to fall after surprisingly
low profit announcements.

THE PRACTICING MANAGER:
Implications for Investing
1. How valuable are published reports by
investment advisors?
2. Should you be skeptical of hot tips?
3. Do stock prices always rise when there is
good news?
4. Efficient Markets prescription for investor

• How valuable are published reports by
investment advisors?

– YES. The EMH indicates that you should be
skeptical of hot tips since, if the stock market
is efficient, it has already priced the hot tip
stock so that its expected return will equal the
equilibrium return.
– Thus, the hot tip is not particularly valuable
and will not enable you to earn an abnormally
high return.

– As soon as the information hits the street, the
unexploited profit opportunity it creates will
be quickly eliminated.
– The stock’s price will already reflect the
information, and you should expect to realize
only the equilibrium return.

• Do stock prices always rise when there is
good news?
– NO. In an efficient market, stock prices will respond to
announcements only when the information being
announced is new and unexpected.
– So, if good news was expected (or as good as
expected), there will be no stock price response.
– And, if good news was unexpected (or not as good as
expected), there will be a stock price response.

• Efficient Markets prescription
for investor
– Investors should not try to outguess the market
by constantly buying and selling securities.
This process does nothing but incur
commissions costs on each trade.

• Efficient Markets prescription
for investor
– Instead, the investor should pursue a “buy and
hold” strategy—purchase stocks and hold them
for long periods of time. This will lead to the
same returns, on average, but the investor’s
net profits will be higher because fewer
brokerage commissions will have to be paid.

• Efficient Markets prescription for investor
– It is frequently a sensible strategy for a small investor,
whose costs of managing a portfolio may be high
relative to its size, to buy into a mutual fund rather than
individual stocks. Because the EMH indicates that no
mutual fund can consistently outperform the market, an
investor should not buy into one that has high
management fees or that pays sales commissions to
brokers but rather should purchase a no-load
(commission-free) mutual fund that has low
management fees.

Case: Any Efficient Markets Lessons from Black
Monday of 1987 and the Tech Crash of 2000?
• Does any version of Efficient Markets Hypothesis
(EMH) hold in light of sudden or dramatic
market declines?
• Strong version EMH?
• Weaker version EMH?
• A bubble is a situation in which the price of an
asset differs from its fundamental market value?
• Can bubbles be rational?
• Role of behavioral finance

Behavioral Finance
• Dissatisfaction with using the EMH to explain events like
1987’s Black Monday gave rise to the new field of
behavioral finance, in which concepts from psychology,
sociology, and other social sciences are applied to
understand the behavior of securities prices
• EMH suggests that “smart money” would engage in short
sales to combat overpriced securities, yet short sale
volume is low, leading to behavior theories about
“loss aversion”
• Other behavior analysis points to investor overconfidence
as perpetuating stock price bubbles

Chapter Summary
• The Efficient Market Hypothesis: We
examined the theory of how both old and
new information are expected to be
incorporated into current stock prices.
• Evidence on the Efficient Market
Hypothesis: We looked at evidence for
various tests of the hypothesis and how
well the hypothesis holds.

Chapter Summary
• Behavioral Finance: We also examined
another important area of research to
explain how stock prices are formed based
on psychological factors affecting investors.

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